60 GEOLOGY OF THE HIGH PLATEAUS. 



iEtna-like summits or craters are visible, and it is doubtful whether the 

 method of eruption was generally such as would generate mountains 

 of that character ; for the larger deluges appear to have emanated from 

 fissures located within restricted areas. Yet apparently some piles of 

 important magnitude were reared by the successive superposition of coulees 

 around a central vent or pipe, and still bear evidences of their origin, though 

 they have been reduced to mere remnants by the wear of ages. 



In the southern part of the district several foci of eruption are discern- 

 ible. The most important was just east of the old andesitic center. From 

 this one emanated the dark trachytic masses wliich have built up a great 

 portion of the Aquarius. Another was situated at the soutliern base of 

 the Tushar, and disgorged the masses which built the southern portion of 

 that range. A line of vents stretched southwest from the 'J'ushar along 

 the western crest of the Markagunt, and sheeted over the greater part of 

 that plateau. Still another occupied the position of Mount Hilgard, at the 

 extreme eastern boundary of the High Plateaus, and a chain of vents 

 stretched southward from it to Thousand Lake Mountain. Around the out- 

 skirts of the more compact inner district many minor eruptions occurred, 

 overflowing numerous outlying patches. 



The rhyolitic eruptions occur chiefly in the Tushar, the Pavant, and 

 Markdgunt — in a word, belong to the western margin of the district. Their 

 grandest masses are displayed in the northern portion of the Tushar. They 

 form the summits of this range, standing in high peaks, which are the 

 loftiest in Utah, excepting two or three in the Uintas. Here no other erup- 

 tive rocks are associated with them, except a few small outbreaks of basalt 

 which overlie them. The platform upon which they lie consists of meta- 

 morphic Jurassic sandstone, upon the eroded surface of which they were 

 outpoured. We find here evidence that the eruptions di-d not occur in 

 rapid succession, but were separated by intervals of time sufiicient to 

 accomplish much erosion. Old valleys scored in the older lavas were filled 

 up by later floods, which were, in turn, chasmed with ravines, revealing 

 the contacts, and this process was repeated again and again. 



Two groups of rhyolitic rocks may be discerned in this locality, each 

 presenting great variety in the texture, as is always the case with rhyolites, 



